1. Field of the Invention
The present invention concerns a method and system in which multiple slices of a subject are excited simultaneously or in parallel in order to generate MR image data for these slices.
2. Description of the Prior Art
U.S. Pat. No. 7,034,530 B2 describes a method in which respectively only a single slice is excited or read out at a given point in time. However, in order to implement the method as quickly as possible MR sequences for different slices are executed interleaved in successive time intervals, whereby overlapping relaxation times and echo times of the respective slices occur. However, this method does not operate to acquire slices in parallel.
U.S. Pat. No. 7,622,925 B2 and U.S. Pat. No. 7,741,842 B2 each describe an accelerated parallel readout technique, wherein in spite of incomplete scanning of k-space the entire data set in k-space is reconstructed by the use of multiple RF acquisition coils.
An additional possibility known for the acceleration of the generation of MR image data is to scan multiple slices simultaneously in a single or, respectively, common excitation and readout step. This method corresponds to an analogous method in computer tomography (CT) in which multiple slices are likewise acquired simultaneously.
In the corresponding MR method, given a switched linear gradient (GZ) a single RF pulse is radiated along the direction of the slice stack in a predetermined of segment 24, whereby RF energy is applied simultaneously across multiple frequency bands in order to excite the spins in multiple parallel slices at the same time (as is shown in FIG. 1). The RF amplitude of the RF pulse is also shown over the frequency in FIG. 1, whereby eight slices S1 through S8 are selectively excited simultaneously.
However, in this method the problem occurs that the echo signals of the different slices overlap both in the time range and in the frequency range, so it is necessary to eliminate the aliasing that occurs.
For this the methods of Multiband GRAPPA (“Use of Multicoil Arrays for Separation of Signal from Multiple Slices Simultaneously Excited”, D. J. Larkman et al., Journal of Magnetic Resonance Imaging 13; Pages 313-317, 2001) and Wideband MRI (“Simultaneous multislice imaging with slice-multiplexed RF pulses”, J. B. Weaver, Magnetic Resonance in Medicine 8, Pages 275-284, 1988) are known according to the prior art.
In GRAPPA the slices S1-S4 that are to be acquired simultaneously are arranged spatially separately from one another such that the signals from the respective slice can thereby be separated from the signals of the other slices in that an RF reception antenna A1-A4 associated with the respective slice S1-S4 is used (as is shown in FIG. 2). Signals in the same frequency band are in fact acquired from the different slices S1-S4 during the readout of the signals from said different slices S1-S4. However, since the slice interval dZ between two adjacent slices in cooperation with the sensitivity profiles of the RF antennas A1-A4 is sufficiently large, it is ensured that the respective antenna A1-A4 essentially detects only the signals of the slice S1-S4 associated with it.
A disadvantage of GRAPPA is that the interval between the adjacent slices S1 through S4 is relatively large, such that the number of slices that are to be acquired simultaneously is relatively low (3) given the imaging of a human heart, for example. Downsizing the RF antennas would not solve this problem since, although the spatial sensitivity of the antenna then would be improved, only a small volume in immediate proximity to the smaller RF antenna would be acquired, such that portions in the middle of the slice (and therefore further distant from the corresponding RF antenna) could only be insufficiently acquired by the RF antenna.
While GRAPPA operates with a spatial separation of the slices S1 through S4 to be acquired, the method according to Weaver operates with a spectral separation of the slices S1 through S4 to be acquired. During the readout a second gradient coding the frequency (slice selection gradient) along the direction of the slice stack is switched simultaneously and perpendicular to the gradient coding the frequency (which gradient is also normally present). The signals of the slices to be acquired are thereby separated into different frequency bands (as is shown in FIG. 3).
It is apparent in FIG. 3 that the frequency bands B of the individual slices S1-S4 do not overlap. This is also necessary (i.e. the frequency interval df between two adjacent slices must be correspondingly large) so that the measurement signals can be associated with the respective slice upon readout. So that the frequency interval df between two adjacent slices is sufficiently large, given a predetermined strength of the slice selection gradients and the frequency coding gradients the slice interval dZ must be selected to be correspondingly large.
The problem in the method according to Weaver is that the slice selection gradient and the readout gradient (“readout”) are simultaneously active upon readout. Two gradients coding the frequency are thereby simultaneously active, which disadvantageously leads to the situation that what is known as the pixel sensitivity profile is tilted proportional to the ratio of the strengths of the two gradients. In order to avoid a blurring of the image to be created, this ratio should be smaller than 1, which corresponds to a flip angle of 45°. (This means that the slice selection gradient should not be stronger than the readout gradient.) Moreover, the method from Weaver is limited by the maximum strength of gradient fields that is to be achieved. In practice this leads to the situation that three slices can be acquired simultaneously in acquisitions of the human head, and at most four slices can be acquired simultaneously in acquisitions of the human thorax or of human extremities. Given the acquisition of the human heart wherein the volume segment to be acquired typically extends across 12-15 cm in the slice stack direction, at most only two slices can be acquired simultaneously with the method by Weaver.